1. Field of the Invention
The present invention relates to an analog signal detecting circuit, and more particularly, to an analog signal detecting circuit that can reliably detect an abnormal signal by simultaneously detecting a magnitude and a frequency of an analog signal.
2. Description of the Related Art
The present invention relates to an analog signal detecting circuit that can reliably detect an analog signal of a fault current such as a leakage current or a short current for an earth leakage breaker or a circuit breaker. In this analog signal detecting circuit, an example of the related art will be described with reference to FIGS. 1 and 2.
As seen with reference to FIG. 1 which is a block diagram illustrating a configuration of an analog signal detecting circuit according to an example of the related art, the analog signal detecting circuit according to the related art includes an analog-to-digital converter 10, a root mean square (RMS) detector 20, a frequency detector 30, and a signal processor 40.
The analog-to-digital converter 10 converts an analog signal to a digital signal.
The RMS detector 20 takes an average of root mean square values, that is, an average of squared values for the digital signals output from the analog-to-digital converter 10 for a predetermined time and calculates a value obtained by taking root for the average value of the squared values.
The frequency detector 30 can calculate a frequency by counting the number of logic high, that is 1 or logic low, that is 0 of the digital signal output from the analog-to-digital converter 10 for a predetermined time and then dividing the number counted by time, and can calculate a cycle by using a cycle-frequency relation equation, which is well known, on the basis of the calculated frequency.
  T  =      1          2      ⁢      π      ⁢                          ⁢      f      
In this case, T is the cycle, and f is the frequency.
The root mean square value (magnitude of the analog signal) detected from the RMS detector 20 and the frequency and the cycle, which are detected from the frequency detector 30, can be delivered to the signal processor 40 at the next stage and processed by being compared with a predetermined reference value by the signal processor 40 on the basis of the magnitude (root mean square value) and the frequency of the analog signal.
However, the aforementioned example of the related art has a problem in that it is not suitable for the earth leakage circuit breaker or the circuit breaker due to high cost and its magnitude which is not small.
Also, the RMS detector 20 needs a stabilizing time for detecting an exact signal value, and the earth leakage circuit breaker or the circuit breaker should break a circuit within a predetermined instantaneous operation time defined by a standard. In this case, a problem may occur in that current breaking of a leakage current or a short current can be failed within a required instantaneous operation time due to the stabilizing time.
Meanwhile, an analog signal detecting circuit according to another example of the related art as shown in FIG. 2 includes a current transformer 1 and a peak detecting circuit 2.
The current transformer 1 detects a current flowing through a circuit as an analog signal and provides the detected current to the peak detecting circuit 2.
The peak detecting circuit 2 includes a half-wave rectifier and a capacitor for smoothing, rectifies and smooths a detected current, and compares the detected current having a DC value with an abnormal determination reference value which is previously set. If the abnormal determination reference value exceeds the DC value of the detected current, the peak detecting circuit 2 outputs a trip control signal to a trip mechanism 3a. 
If the trip mechanism 3a is operated to a trigger position by the trip control signal, a switching mechanism 3b interlocked to the trip mechanism 3a can perform a trip operation to a circuit breaking position to protect an electric load from an abnormal current (fault current) such as an earth leakage current or a short current.
However, the analog signal detecting circuit according to another example of the related art as shown in FIG. 2 has a problem in that the peak detecting circuit 2 fails to rectify and smooth a detecting current if a rapidly great current as the detecting current flows to the circuit within a short time due to thunderbolt, etc.